Continuous time delta sigma modulators comprise a continuous-time loop filter rather than the more common switched-capacitor filter. Continuous-time delta-sigma modulators offer inherent protection against signal aliasing and less noisy virtual ground nodes at the input. A further advantage is that modulator clock rates can be raised, because settling time restrictions are relaxed. Resonance frequencies and quality factors of resonators in continuous time, bandpass resonators are subject to process and temperature variations. The behaviour of a DSM is very sensitive to these parameters. Therefore, they need to be calibrated, at least once during the manufacturing process, or even on a regular basis.
European Patent Application EP 1 933 460 A1 describes an analogue RF device for wireless communication. The device includes, inter alia, a tuneable bandpass filter which receives test signals from a controller and reacts by returning scattered signals to the controller. The controller determines a frequency dependence of a scattering parameter of the bandpass filter. The controller adjusts the tuneable bandpass filter in order to tune the bandpass filter to a desired frequency dependence of at least one scattering parameter. Measuring scattering parameters over a frequency range is, however, a complicated task and requires a high degree of accuracy, as well as appropriate equipment. For many applications this would be prohibitive. Usually, the determination of the resonance frequency and the quality factor of the bandpass filter is sufficient for a large number of applications. The entire disclosure of EP 1 933 460 A1 is hereby incorporated by reference into the description.
U.S. Pat. No. 7,193,546 (issued to Melanson, assigned to Cirrus Logic, Inc.) discloses a phase-measuring delta-sigma modulator calibration method and apparatus. The apparatus disclosed in the '546 patent comprises a signal generator that is external to the delta-sigma modulator. The signal generator output is injected into the quantizer of the delta-sigma modulator as an artificial quantization error. By direct comparison of the output of the delta-sigma modulator with the output of the signal generator it is possible to determine the response of the modulator to quantization noise, or in other words, measure the noise transfer function (NTF). The method according to the '546 patent does not disclose that a component of the delta-sigma modulator, such as a resonator, is caused to oscillate and that the frequency of the oscillation is compared to a reference frequency. The entire disclosure of U.S. Pat. No. 7,193,546 is hereby incorporated by reference into the description.